Retrograde Motion

One phenomenon that aincient astronomers had difficulty
explaining was the retrograde motion of the planets.
Over the course of a single night, a planet will move from
East to West across the sky, like any other celestial object
near the ecliptic.

(Most objects in our sky appear
to rise somewhere on the Eastern horizon and set somewhere
on the Western horizon. The only exceptions are stars near
the North celestial pole, that stay above the horizon all the time
and appear to make counterclockwise circles around the
celestial pole. As one travels further North, the region of
the sky that remains above the horizon at all times becomes
larger, until the entire sky appears -to an observer at the
North Pole- to be simply circling the North star. As one
ravels South, the region that remains above the horizon
becomes smaller, diminishing
to zero size for an observer on the Equator. If one continues
South of the Equator, one would observe a progressively larger
region surrounding the South celestial pole that
remains above the horizon at all times. Stars in that region
would appear to circle the South celestial pole in clockwise
circles.)

If observed from one night to the next, however, a planet
appears to move from West to East against the background
stars most of the time.
Occasionally, however, the planet's motion will appear to reverse
direction, and the planet will, for a short time, move from
East to West against the background constellations. This reversal
is known as retrograde motion, and is illustrated in the
following animation.

Apparent Motion of Mars Against Background Stars

Ptolemaic Explanation

The model of the solar system developed by Ptolemy (87 - 150 A.D.) was a
refinement of Aristotle's (384 - 322 B.C.) universe. This model consisted
of a series of concentric spheres, with the Earth
at the center (geocentric). The motions of the Sun, Moon, and
stars was based on perfect circles. To account for the observed retrograde
motion of the planets, it was necessary to resort to a system of
epicycles, whereby the planets moved around small circular paths that
in turn moved around larger circular orbits around the Earth. This accounts
for retrograde motion, as shown in the animation below:

Retrograde Motion in the Ptolemaic (Geocentric) System

In its final form, the model was extremely complicated, requiring
many nested levels of epicycles, and with even the major orbits offset
so that they were no longer truly centered on the Earth. Despite all of
this fine tuning, there remained significant discrepancies between the
actual positions of the planets and those predicted by the model.
Nevertheless, it was the most accurate model available, and it remained
the accepted theory for over 13 centuries, before it was finally replaced
by the model of Copernicus.

Copernican Explanation

Copernicus replaced the geocentric universe of Ptolemy with one that
was centered on the Sun (heliocentric), with only the Moon orbiting
the Earth. His model was still based on circular orbits (and therefore
still required further refinement), but it was able to achieve superior
precision than the Ptolemaic model without the need for epicycles or other
complications. The explanation for retrograde motion in this system arises
from the fact that the planets further from the sun are moving more slowly in
their orbits than those closer to the sun. The retrograde motion of Mars
occurs when the Earth passes by the slower moving Mars, as shown in the
following animation:

Retrograde Motion in the Copernican (Heliocentric) System

When combined with the refinements of Kepler (elliptical orbits with
the sun at one focus, relationships between distance from sun and orbital
speed - both within a single orbit and between orbits) this does, in fact,
provide the correct explanation for the observed retrograde motion along
with precise predictions of the positions of the planets.

For additional discussion of retrograde motion and the Ptolemaic and
Copernican models of the solar system, you may want to check out the
following web sites: